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Summary of Restricted Stock Unit's Activity (Detail) - Restricted Stock Units (RSUs) - $ / shares
12 Months Ended
Jan. 31, 2019
Jan. 31, 2018
Jan. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Non-vested awards outstanding at beginning of year, Shares 1,091,500    
Granted, Shares 571,500    
Vested, Shares (214,166) 0 0
Forfeited, Shares (70,000)    
Non-vested awards outstanding at end of year, Shares 1,378,834 1,091,500  
Non-vested awards outstanding at beginning of year, Weighted average fair value $ 26.92    
Granted, Weighted average fair value 36.55 $ 26.01 $ 28.10
Vested, Weighted average fair value 26.86    
Forfeited, Weighted average fair value 29.14    
Non-vested awards outstanding at end of year, Weighted average fair value $ 30.82 $ 26.92